With ever-increasing demand for “green” energy, solar power has drawn a lot of attention by its rapid growth in recent years. To convert the fluctuating direct current (DC) output voltage from solar modules into a well-regulated sinusoidal alternating current (AC) voltage, the architecture of a typical solar power conversion system is either two-stage or single-stage, with or without, DC/DC converter. The existence of a DC/DC stage can maintain the input voltage of an inverter at a constant and controlled level, and decouple the control of voltage and power flow. The inverter transforms DC power from the photovoltaic array (PV) array to grid-quality AC power. Depending upon the system architecture, the inverter may also charge and discharge energy storage, and may control smart loads, e.g. smart appliances, especially in residential systems. The inverter/controller, if it contains adaptive logic, may also determine when excess energy is dispatched to the grid or stored.
Some design goals for inverter topologies are maximum power point (MPP) tracking of the solar panel for detection of the input voltage with the maximum input power, and maximum energy efficiency for the inverter. In order to run the inverter at the MPP, the circuit has to be able to adjust the input voltage according to the current light conditions. The MPP is usually at approximately 70% of the open loop voltage, but this is also dependent on the selected panel technology. To achieve this, the input voltage can be adjusted dynamically, for example with a boost circuit. In a 2nd stage, the DC-voltage can be inverted into a sinusoidal grid-compatible voltage. The booster adjusts the input voltage to the MPP. The output inverter injects the sinusoidal output current into the power grid.
Florent Boico et. al: “Solar battery chargers for NiMH batteries”, IEEE transactions on power electronics, Vol. 22, No. 5, September 2007, discloses new voltage and temperature-based charge control techniques. To increase charge speed, an MPP tracker (MPPT) is implemented within a micro-controller using a Sepic converter and a bypass switch. The Sepic topology was used because it offers common ground between the input and output and continuous current at the input. The bypass switch is controlled by the microcontroller. MPP is achieved by adjusting a DC/DC converter control loop, When the algorithm has stabilized around the MPP, the micro-controller assesses whether MPPT increases delivered power or not. This is done by switching to direct connection and comparing the charging current delivered to the battery. The best solution will be retained. After a predetermined time period, the two possibilities (direct connection or MPPT) are tried again in case the MPP of the solar panel changed due to light intensity or temperature.